Since a lithium secondary battery has a high energy density in comparison with another secondary battery, the lithium secondary battery can be downsized and reduced in weight. In this point of view, the lithium secondary battery is frequently utilized as an electric source of a mobile electric device such as a cellular phone, a personal computer, a personal digital assistant (PDA) and a handy video camera and in the future, the demand as the electric source is promising to be increased more than ever.
In order to cope with energy problem and environmental problem, moreover, an electric vehicle or a hybrid electric vehicle with a combination of a motor driven by a nickel hydride battery and a gasoline engine are developed and diffused conspicuously. In this vehicle use, the high performance of the battery to be used is required to be developed more than ever and thus, attention is paid to the lithium secondary battery.
In the lithium secondary battery, various carbon materials with excellent safety and lifetime are generally used as the negative electrode active material. Among the carbon materials, graphite can be obtained at a high temperature at least more than 2000° C., normally within a range of about 2600 to 3000° C. and is excellent material because of having a high energy density, but has some problems in high input/output characteristics and cycle characteristic. In this point of view, for the use of the electric vehicle and the storage battery requiring the high input/output characteristics, for example, low crystalline carbon with low degree of graphitization, which is fired at a lower temperature in comparison with graphite, is mainly researched and used.
In order to respond to the requirement of the high performance of the electric vehicle, recently, the lithium secondary battery is required to be highly developed in performance, which calls for urgent attention. With regard to the characteristics of the lithium secondary battery, the electric potential in the side of the negative electrode is sufficiently reduced to enhance the actual battery voltage and thus exhibit the highly and sufficiently output characteristic.
Moreover, the discharge capacity of the lithium secondary battery is an important characteristic in view of the sufficient supply of a current which is an energy source for the electric vehicle. Furthermore, the ratio of the charging capacity to the discharging capacity, that is, the initial efficiency is required to be set higher in view of the larger amount of discharging current in comparison with the amount of charging current.
In addition, it is preferable that the lithium secondary battery maintains a higher charging capacity even at high current density so as to realize the charging process in a short period of time so that the capacity retention rate of the lithium secondary battery is required to be developed. Namely, it is required for the lithium secondary battery that the output characteristic, the discharging capacity, the initial efficiency and the capacity retention rate are developed under the proper balance condition.
In order to achieve the aforementioned requirement, as the negative electrode active material of the lithium secondary battery are intensely researched and developed carbon materials such as coke or graphite. In this case, the discharge capacity can be developed, but the initial efficiency cannot be sufficiently developed. Moreover, the actual battery voltage is not sufficient so as not to satisfy the requirement for the high output characteristic and the capacity retention rate.
In Reference 1, for example, as the negative electrode material utilizing intercalation or doping is disclosed a carbonaceous material defined in specific surface and X-ray diffraction crystal thickness, the carbonaceous material being obtained through the thermal decomposition or carbonization of an organic compound. However, the thus obtained negative electrode material is not sufficient for the use of a vehicle such as the hybrid electric vehicle.
In Reference 2, as the negative electrode material is disclosed a carbon material with excellent cycle characteristic and higher discharge capacity, the carbon material being obtained through the thermal treatment for calcined coke as a raw material to remove impurities therefrom under a non-reactive atmosphere. However, the thus obtained negative electrode material is not sufficient in output characteristic and the like for the use of vehicle installation such as the hybrid electric vehicle.
In Reference 3, as the negative electrode material is disclosed a carbon material, the carbon material being obtained through the thermal treatment for a carbonaceous material with a specific cover layer which the carbonaceous material has a crystalline structure similar to that of graphite. In Reference 4, as the negative electrode material is disclosed a carbon material with high discharge capacity, the carbon material being obtained through the thermal treatment at low temperature for coke as a raw material under a non-reactive atmosphere to remove impurities therefrom. However, both of the carbon materials are not sufficient in battery characteristics for the use of vehicle installation such as the hybrid electric vehicle.
In Reference 5, as the negative electrode material is disclosed a thermally treated coke for providing a lithium secondary battery with high charge capacity and discharge capacity, the coke being obtained through the thermal treatment within a temperature range of 500 to 850° C. for a green coke derive from a petroleum or a coal. However, the coke is not sufficient in output characteristic for the use of vehicle installation such as the hybrid electric vehicle.
The research and development of the negative electrode material of the lithium secondary battery made of the low crystalline carbon material using the coke or the like as the raw material is directed at the improvement in characteristic of the negative electrode material of the lithium secondary battery for the use of an electric source for a small mobile instruments, but, as of now, the research and development of the negative electrode material of the lithium secondary battery is not directed at the enhancement in characteristic of the negative electrode material of the lithium secondary battery with large current input/output characteristics suitable for the use of an electric source for the hybrid electric vehicle.